a wide range of salt tolerance exists in the plant kingdom some species are extreme halophytes and ecologically competitive only under saline conditions. others, the glycophytes, are salt-sensitive plants that grow well under salt-free conditions. this diversity of salt tolerance has evolved over millions of years. for several thousands of years, domesticated crop species typically have been provided with optimum conditions to obtain high yields. this has selected against those characteristics that help plants adapt to extreme environments (rosielle and hamblin 1981). thus, a narrower range of tolerance to salt and other environmental stresses exists among crops than for wild species. indeed, many crops are moderately to highly sensitive to salinity. this chapter focuses on the salt tolerances of crops and the improvements that can be made through conventional plant breeding strategies or the application of new approaches in molecular biology. historically, salinization of a crop production area has resulted in the replacement of salt-sensitive species with more tolerant ones. methods that leached salts from soils and management practices that avoided salinity problems had yet to be developed (marr 1967). a few highly salt-tolerant crops, such as sugar beet, barley, cotton, asparagus, and sugarcane, could substitute for more sensitive species. such substitution is still practiced in the world's saline growing areas. however, climatic, nutritional, and economic needs often call for a greater diversity of salt-tolerant crop species than is available. only recently has man bec